I would love both a ground and a flying race. I already once opted for a race in AQ40. I would be epic to do a race there. Especially on those scarab mounts, who dismount you if you take damage = more exciting race. Also no speedbuffs should be enabled/available unless like with any kartgame, some shoes would appear and make you go faster.
Same with flying mounts basically but then around Uldum or something in the clouds.
And who cares if there are race-specific mounts. Just hop onto your own mount tbh.
I'd love mount races. They could do something similar to the cloud race you do as a quest for the Order of the Cloud Serpent, just cooler. Ground races could also be a ton of fun if designed properly - with proper race tracks, obstacles, temporary buffs etc, just like a BG or scenario. You could earn a racing prize currency over time and turn it in for extra cool mounts.
Nothing for :
Humans: They can ride the light.
Worgens: Do what they did with the horsey mount and give them a boring armourless Gryphon (omg so inventive)
Draenei: Mini exodars.
Undead: Undead something. Undead bats?
Taurens: Attach wings to a kodo and hope for the best
Goblins: Something like an engi helicopter or they can jut have a cannon that fires them a great distance.